Focused ion beam (FIB) technology is an important tool for the semiconductor industry. Focused ion beams are used for failure analysis, transmission electron microscopy specimen preparation, and circuit and mask modification. FIB micro and nanofabrication can be utilized to reduce the complexity required in conventional fabrication technology, in particular lithography, etching and implantation, which has to satisfy various requirements for different components fabricated on the same substrate.
The success of FIB technology is due to the invention of liquid metal ion sources (LMIS) and liquid metal alloy ion sources (LMAIS), respectively. In the following, reference to only LMIS or only LMAIS, respectively, should also be understood as a reference to the other type of ion source unless it is apparent that only either LMIS or LMAIS is meant. Three basic designs of LMIS exist, needle-type emitters, capillary-type emitters and porous emitters. For comparison, a photograph of a needle-type emitter and a capillary-type emitter is shown in FIG. 7.
In a needle-type emitter, a tiny hairpin (needle tip) and a filament, typically made of W, Ta, Ti or Ni, are used as an emitter (see left-hand side of FIG. 7). The emitter is wetted and loaded with the liquid metal source material to provide a liquid metal reservoir. For wetting of the source, the source material must be provided in liquid form. To this end, a resistance heater or an electron beam heater may be used. Then, the emitter is dipped into the heated liquid metal in high vacuum (about 107 Torr). During operation, electric current is supplied to the filaments which are thus resistively heated. The heated liquid metal flows towards the needle-tip. High voltage is then applied between the needle tip and an extraction electrode. Due to the high electric field strength at the needle tip, an even smaller tip of liquid source material forms a so-called Taylor cone from which the ions are emitted. Thereby, a stable ion beam is generated from the source material. It is apparent that the tip end of the needle tip should form the hottest spot of the emitter so that liquid metal ions are produced essentially at the tip end.
In the following, the needle-type emitters will be explained in more detail in with reference to FIGS. 8A and 8B. FIGS. 8A and 8B are schematic views of a prior art needle-type liquid metal ion source. The emitter includes a needle tip 10, a filament 11 and supports 50. Typically, the supports 50 are mounted to a ceramic base and provide an electric terminal for filament 11. Filament 11 extends between the two supports and has an arc-like shape. At the apex of the arc, the needle tip 10 is attached to filament 11. As shown in FIG. 8B, needle tip 10 is attached to filament 11 by welding techniques, e.g. electrical spot-welding, so that weld spots 15 are formed. Therefore, the heating of needle tip 10 is only indirect in that the heat must be transferred from filament 11 to needle tip 10 via weld spots 15. As a result, it is not ensured that the tip end of the needle tip 10 is really the hottest part of the emitter. This may reduce the efficiency of the emitter. Furthermore, the reservoir portion of needle tip 10 may be excessively heated so that liquid metal material evaporates from the reservoir. Thus, the life time of the ion source is reduced and contamination of the specimen and the ion beam apparatus is increased.
A second type of LMIS is known as capillary-type or reservoir-type emitters. An example of such a capillary-type emitter is shown on the right-hand side of FIG. 7 and in FIG. 9. For the capillary design, the emitter module 10 consists of two metallic plates with a small source material reservoir 30. A sharp blade is accurately machined on one side of each plate. A thin layer of material is sputter-deposited on the other three sides of one of the plates, to act as a spacer; when the two emitter halves are tightly clamped together, a slit 17 of about 1 μm is left between the blades. Furthermore, a heater 40 is provided for heating the source material so that it is in a liquid state. The liquid source material flows through this tiny channel 17, forming a free surface at the exit of the slit 17 with a radius of curvature in the order of 1 μm. Under a strong electric field generated by the application of a voltage difference between the emitter and an extraction electrode located directly in front of it, the free surface of the liquid metal approaches a condition of local instability, due to the combined effects of the electrostatic force and the surface tension. A protruding cusp, i.e. a Taylor cone, is created. When the electric field reaches a value of about 109 V/m, the atoms at the tip spontaneously ionize and a thrust-producing ion jet is extracted by the electric field, the evaporated atoms from the tip being ionized while the electrons are rejected in the bulk of the liquid metal by tunneling.
However, the structure and manufacturing of such a capillary-type emitter is relatively complex. Furthermore, high heating currents are required by capillary-type emitters
It is therefore desirable to provide an improved emitter for a liquid metal ion source or a liquid metal alloy ion source.